1. Field of the Invention
The present invention relates generally to a method of producing hollow articles of thermoplastic materials. More particularly, the invention relates to a method in which initially a compact blank, such as, a tablet, a pellet, a disk or the like, is formed by injection molding in a mold cavity. The blank may be composed of several layers of different plastics material components which surround one another. The blank is subsequently subjected to a compression molding operation in the heated state in which its thickness is reduced and its surface area size is increased. Finally, the hollow article is formed from the compressed blank again in the heated state in a mold by a stretching and blow molding operation.
2. Description of the Prior Art
In the production of hollow articles, particularly containers having large volumes, it is already known in the art to process prepared plastics material blanks initially in the heated state by a forging operation between die surfaces and, subsequently, to produce from the blank prepared in this manner a hollow article by means of a hot-forming or blow molding operation. This is described, for example, in Federal Republic of Germany Offenlegungsschrift No. 21 40 341.
As is the case in German Democratic Republic patent No. 58 373, the blank can also be prepared by means of an injection molding operation, as disclosed in Federal Republic of Germany Offenlegungsschrift No. 24 17 270.
The injection molded blanks to be deformed between the die surfaces initially have the shape of a relatively thick tablet, pellet or disk, which is subsequently reduced in its thickness by the compression molding operation and its structure is condensed and, simultaneously, its surface area size is increased.
The plastics material blanks prepared by injection molding can be further deformed into relatively thin layers while still in the initial heated state, i.e., immediately following the injection molding operation, by compressing the blanks with biaxial orientation. These layers can then be further deformed into the actual hollow articles by a stretching and blow molding operation.
It is also known in the art to use blanks to be deformed by a compression molding operation which have a multilayer composition made of different plastics material components. The blanks may have multiple layers with varied combinations of layers. The different layer combinations lead to different difficulties with respect to manufacture of the blanks.
The thicknesses of the layers of the individual plastics material components are determined basically by the given volumetric flows of the individual material components. However, the distribution of the material components in radial flow direction depends to a significant extent from the flow properties and the flow distances present in each case. The flow properties of the individual material components are determined by the viscosity and the melt temperature level. In addition, the wall temperatures of the injection molding tool have a substantial influence on the distribution of layer thicknesses during the non-stationary filling procedure. For example, if the viscosities of several different material components are the same under a given shearing load, a parabolic rate of flow pattern develops. On the other hand, different viscosities of the individual material components result in a combined rate of flow pattern having, for example, a discontinuous parabolic shape.
When the temperatures in the walls of the two injection molding halves are not equal, a uniform layer thickness is not formed in one and the same material component. Rather, one layer of this material component may be thinner than the other layer, so that an intermediate layer of another material component may be shifted in an undesirable manner from its predetermined center position.
The layer configurations of the plastics material blanks described above are in most cases undesirable because this substantially impairs the quality of the hollow articles to be manufactured by the stretching and blow molding operation.
These disadvantages can also not be eliminated by the compression molding and condensing operations in which the blanks are formed into compressed articles having larger surface area size prior to the stretching and blow molding operation.
It has been found that significant difficulties occur in the manufacture of compact blanks by injection molding from thermoplastic materials especially in those cases when the blanks have a relatively large volume and, simultaneously, must have a multiple layer structure, and when the blanks are to be suitable for a stretching and blow molding operation of relatively large hollow articles. In this connection, the danger exists that it is not possible to obtain the desired distribution of the plastics material conducted through the injection ducts into the mold cavity. In other words, an undesirable material layer distribution may result in the blank which would be maintained during the subsequent compression molding operation and the stretching and blow molding operation and which would negatively influence the usefulness of the finished hollow article.
The above-described disadvantage can be avoided, as indicated in Federal Republic of German Offenlegungsschrift No. 21 40 341, if the blanks to be processed between the dies are prepared by placing several layers of foil blanks of plastics material one on top of the other. However, this type of manufacture of the blanks is relatively cumbersome and expensive. In addition, plastics material blanks which are formed from foil layers and are processed between dies cannot in all cases be used for stretching and blow molding of hollow articles having relatively large volumes.
It is, therefore, the primary object of the present invention to provide a method and apparatus for the manufacture of hollow articles of thermoplastic materials in which the disadvantages mentioned above are avoided.
It is a further object of the invention to provide a method of manufacturing hollow articles of thermoplastics materials in which an exact material distribution in the mold cavity is ensured even when the blanks have to be composed of several layers of different plastics components.
It is yet another object of the present invention to optimize the layer structure of the blanks obtained by injection molding, in an operation for deforming the blanks into a compressed blank having reduced thickness and, simultaneously, an increased surface area size, for a subsequent stretching and blow molding operation during which the thickness is rendered uniform.